In a traveling-wire electroerosion (i.e. EDM or ECDM) machine, an electrically conductive wire composed of, say, copper or brass, serves as a cutting tool electrode. The continuous wire electrode is stretched between a pair of guide members defining a straight line path of wire travel in a cutting zone and is advanced axially to traverse a workpiece. An electrical machining current, typically in the form of a succession of electrical pulses, is passed between the traveling wire electrode and the workpiece to electroerosively remove material from the latter while relative displacement between the traveling wire electrode and the workpiece is effected along a prescribed path to form a cut of the corresponding contour in the workpiece.
The wire electrode is continuously renewed in the cutting zone by being continuously dispensed from a wire supply which is typically in the form of a wire storage spool having wire wound thereon. In dispensing the wire, a traction force is applied to the wire leading from the storage spool to unwind the wire therefrom. The traction force is typically produced by an intense capstan and pinch roller drive disposed downstream of the cutting zone to advance the wire therethrough. An intense braking capstan and pinch roller drive is also provided upstream of the cutting zone to make the traveling wire sufficiently taut between the cutting guide members. Because the wire must be as thin as 0.1 to 0.2 mm in diameter and must be composed of a material not to high in strength, there is a limitation in the tension which can be applied to the wire to maintain and increase its linearity.
Heretofore, a wire storage spool having wire wound thereon in weight up to 5 kg has been commonly in use. Such a spool is found to be inadequate where the workpiece is large and/or thick or the cutting path or trajectory is long enough to use up the wire on a single spool, and then a much more massive spool weighing, say, 30 kg is desirable. It has been found, however, that considerable difficulty is encountered in dispensing and feeding out wire as thin as 0.1 or 0.2 mm diameter from such a massive spool smoothly. It has been found that the conventional arrangement with an ordinary spool in which the spool is rotatably supported on a shaft retained in its axial opening and is allowed to rotate to unwind the wire can hardly be utilized with success. The wire being dispensed tends to slacken and jerk.